Variable density window



JDU"J I 1 March 9, 1943. E. H. LAND VARIABLE DENSITY WINDOW Filed Dec. 12. 1940 FIG.

FIG. 2

I W Fr ENTOR Patented Mar. 9, 1943 VARIABLE DENSITY WINDOW Edwin H. Land, Cambridge, Mass., assignor to Polaroid Corporation, Dover, Del., a corporation of Delaware Application December 12, 1940, Serial No. 369,762

Claims. (C1, 88-65) This invention relates to variable density windows.

An object of the invention is to provide a variable density window comprising a plurality of overlying light-polarizing elements and means associated therewith controlling at will the amount of light transmitted thereby.

Another object is to provide such a window wherein the light-polarizing elements are fixedly positioned with respect to each other, and wherein the density of the window as a whole is varied by altering the tension on a sheet of material showing photoelastic effects and positioned between the polarizing elements.

A further object is to provide such a window wherein the polarizing surfaces are secured to or otherwise made a part of the sheet of material showing photoelastic effects.

A still further object is to provide, as a new article of manufacture, a sheet of material showing photoelastic effects under compression or other stress and having polarized material applied to its opposite surfaces, the transmission axes of the polarizing surfaces being at a predetermined angular relation to each other.

Other objects and advantages will in part appear and in 'part be pointed out in the course of the following description of two embodiments of the invention, which are given as non-limiting examples, in connection with the accompanying drawing, in which:

Figure 1 is a sectional view showing a variable density window embodying one form of the invention;

Figure 2 is an elevation of the window shown in Fig. l; and

Figure 3 is a perspective view, partly broken away, of a modification of the invention comprising a sheet of material showing photoelastic effects and coated on both sides with polarizing material.

In its simplest embodiment, the variable density window of the present invention comprises a pair of parallel, overlying polarizing surfaces which are preferably positioned with their transmission axes either relatively parallel or relatively perpendicular. Between said polarizers there is positioned a sheet of material capable of showing photoelastic effects under stress, said material being preferably of such characteristics that under a predetermined compression in a predetermined direction in the plane of the sheet it will function as a half wave plate for the light transmitted by each of said polarizers and possessing such a degree of resiliency that it will return to its initial condition when the compressive force is removed. Thus light passing through each of said polarizers may have its vibration direction altered at will so that it may be either transmitted or absorbed by the other said polarizing element.

Referring to Figs. 1 and 2, element In represents a channeled window frame formed in any conventional manner and providing a window aperture of substantial area. Mounted on one side of frame to, as by means of a suitable frame I2, is a laminated window element I4 comprising a central layer ii of polarizing material, bonded between a pair of protecting plates l6 of glass or other transparent plastic. Similarly mounted in the other side of frame I0 is a second laminated window element I8 comprising a central layer 20 of polarizing material, bonded between a pair of protective plates 22 of glass or other transparent plastic.

Polarizing layers I5 and 20 may conveniently be made from one of the polarizing sheets or films sold under the trade name Polaroid. Window elements l4 and I8 may be positioned so that the transmission axes of polarizers l5 and 20 are at any desired angle to each other. Preferably, however, said elements are so positioned that said polarizing layers have their respective transmission axes relatively perpendicular and at angles of 45 to the vertical, as is indicated in Fig. 2, wherein arrows 24 and 25 represent the transmission axes of polarizers l5 and 20, respectively.

Element 30 represents a sheet or plate of transparent plastic material capable of showing photoelastic efiects under compression or like stress and possessing sufiicient resiliency to return to its normal condition after the stress is removed. Suitable materials which may be used for this purpose include plastic materials, such as glass, transparent cast phenolic resins such as Bakelite; Marblette, Lucite, or polyvinyl acetal, and it will be understood that such materials should preferably possess suflicient rigidity and sheet 30 should be sufficiently thick to prevent buckling under compression in the plane thereof. Plate 30 may be provided at its top and bottom with suitable channeled rims 28, 26, and is fixedly positioned in the top of frame I0 as by means of channel irons 32 or other suitable spacer means. Rim 26 on the lower edge of plate 30 is slidably positioned between suitable spacer elements 34 and is in contact with a suitable cam 35 or other similar device for exerting compressive force thereon. As is shown in Fig. 1, cam 35 is mounted on shaft 36 and is provided with suitable means for controlling its rotation, said means comprising gear 38 meshing with worm 42 controlled by a crank or knob 40 journaled in frame Hi. It will of course be understood, however, that this cam mechanism is merely illustrative of many similar means for exerting force on plate 30 in accordance with the practice of the invention.

It is believed that the operation of the above described window will now be apparent. Plate 30 is preferably of such characteristics that it is optically isotropic when the device is in the position shown in Fig. 1, that is to say, when cam 35 is exerting the least pressure. In this position, therefore, the operation of plate 30 may be disregarded, and inasmuch as polarizers i and are positioned with their transmission axes relatively perpendicular, it will be seen that this is the position of maximum extinction for the window.

As cam is rotated, plate 30 becomes birefringent and imparts a gradually increasing relative retardation to one component of the polarized light transmitted by, for example, polarizer I5. As the relative retardation increases, more and more light is transmitted by polarizer 20. When the compression reaches the degree whereat plate 30 becomes a half wave retardation plate, all of the light transmitted by polarizer I5 has its vibration direction rotated through 90 and thus is transmitted by polarizer 20. It follows that the position of maximum transmission is that wherein plate 30 is under such a degree of compression that it functions as a half wave plate.

The positions of the elements with respect to their axes may be changed to a considerable extent. It will be understood that in order to obtain the maximum density variation in this arrangement, the direction of compression of plate 30 should be at 45 to the axes of the two polarizers, as in the arrangement shown in Figs. 1 and 2. It will of course be obvious that the polarizers may be positioned with their axes parallel, and in this case also the compressive force should be at an angle of 45 to said axes. Various other relative positions of the polarizers and direction of force may also be used, but the positions illustrated are to be preferred.

Fig. 3 shows a modification of the invention wherein the polarizers and birefringent layer are combined in a unitary structure, comprising a sheet or plate of any of the plastic materials mentioned above, coated on each surface with a layer 52, 54 comprising polarizing material. Layers 52 and 54 may conveniently comprise one of the polarizing films or sheets sold under the trade name Polaroid and bonded in any suitable way to the surface of plate 50. More par ticularly, layers 52 and 54 may comprise a highly concentrated, oriented suspension of polarizing particles in a suitable plastic medium, which is applied as a very thin coating to plate 50 and wherein the orientation of the polarizing particles may conveniently be secured by smearing or otherwise stroking the said suspension during the coating operation.

The transmission axes of polarizing layers 52 and 54 may be at any desired angular relation to each other, but as was explained above in connection with polarizers i5 and 20, the preferred arrangement is to have said axes relatively perpendicular, as is indicated by arrows 55 and 56. The particular advantage of this arrangement is that it makes it easier to apply the compressive force in a direction at equal angles to the transmissionaxes of the polarizer. It will'be understood that plate 50 may be substituted in place of plate 30 in the embodiment of the invention shown in Fig. 1, in which case window elements 14 and 18 may either be eliminated entirely or may be replaced by plain protective layers of glass or other transparent plastic. It will also be understood that it is within the scope of the invention to provide layers 52 and 54 with any suitable protective coating such as lacquer or thin sheets of any suitable plastic.

The plastic materials already described as suitable for use in connection with the present invention in the formation of layers 30 or 50, for example, are plastics which have a high stress optical coefficient. This condition is preferred. Furthermore, the plastics employed should show substantially no cold flow when subjected to compression.

It should be understood furthermore that while a preferred embodiment of the device constitutes the provision of means permitting variations from a maximum to a minimum of transmitted light and that such means will ordinarily contemplate the use of a material as the intermediate plastic layer which, when subjected to a predetermined compression, functions as a half-wave retardation plate, such a preferred condition is not essential to an operation of devices falling within the scope of the invention. It may for example be unnecessary or undesirable to have more than a relatively small amount of light transmitted by the combined polarizers and intermediate plastic layer or, on the other hand, it may be found unnecessary to completely block transmitted light. Under these circumstances, the use of an intermediate plastic layer which functions as a wave retardation device less than a half-wave retardation device for example may prove satisfactory. All such modifications of the invention are to be deemed to fall within its scope.

It will be understood that with devices ,such as are shown and described in connection with the present invention, the transmission of the entire device for light incident on either surface will be a function of the compression exerted on the intermediate plastic layer.

Since certain changes may be made in the above construction, and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

l. A variable density window comprising, in combination, frame means providing a window aperture of substantial area, means providing a plurality of light-polarizing elements at least substantially of the area of said aperture, a sheet of light-transmitting, resilient material at least substantially of the area of said aperture and adapted when subjected to a predetermined compressicn in the plane thereof to function as a half wave retardation device, means for mounting said elements within said frame in substantially parallel relation, said sheet being positioned between and substantially overlying said polarizing elements, and releasable means for subjecting at least said sheet to said predetermined compression to vary the amount of light transmitted by said window.

2. A variable density window comprising, in combination, frame means providing a window aperture of substantial area, means providing a plurality of light-polarizing elements at least substantially of the area of said aperture, a sheet of light-transmitting, resilient material at least substantially of the area of said aperture and adapted when subjected to a predetermined compression in the plane thereof to function as a half wave retardation device, means for mounting said elements within said frame in substantially parallel relation, said sheet being positioned between and substantially overlying said polarizing elements, said polarizing elements being positioned with their respective transmission axes relatively perpendicular, and releasable means for subjecting at least said sheet to said predetermined compression in a direction at substantially 45 to the transmission axes of said polarizing elements to vary the amount of light transmitted by said window.

3. A variable density window comprising, in combination, frame means providing a window aperture of substantial area, a plurality of sheetlike, light-polarizing elements at least substantially of the area of said aperture, means for mounting said elements in substantially parallel relation within said frame, said elements being mounted with their respective transmission axes at a predetermined angular relation to each other, a sheet of light-transmitting, resilient material at least substantially of the area of said aperture, said sheet being normally isotropic and being adapted when subjected to a predetermined compression in the plane thereof to function as a half wave retardation device, means for mounting said sheet between and overlying said polarizing elements, and releasable means for subjecting said sheet to such predetermined compression in a direction at substantially equal angles to the transmission axes of said polarizing elements to vary the amount of light transmitted by said window.

4. A variable density window comprising, in combination, frame means providing a window aperture of substantial area, a sheet of lighttransmitting, resilient material at least substanially of the area of said aperture and adapted when subjected to a predetermined compression in the plane thereof to function as a half wave retardation device, means for mounting said sheet within said frame, means providing a lightpolarizing surface on each side of said sheet, and releasable means for subjecting said sheet and the polarizing surfaces associated therewith to such predetermined compression to vary the polarizing surfaces having their respective transmission axes relatively perpendicular, and releasable means for subjecting said sheet and the polarizing surfaces associated therewith to such predetermined compression in a direction at substantially 45 to the transmission axes of said polarizing surfaces to vary the amount of light transmitted by said window.

6. As an article of manufacture, a sheet of light-transmitting, resilient material adapted when subjected to a predetermined compression in the plane thereof to function as a half wave retardation device, and means providing a lightpolarizing surface bonded to each side of said sheet.

7. As an article of manufacture, a sheet of light-transmitting, resilient material adapted when subjected to a predetermined compression in the plane thereof to function as a half wave retardation device, and means providing a lightpolarizing surface bonded'to each side of said sheet, said polarizing surfaces having their respective transmission axes at a predetermined angular relation to each other.

8. A variable density Window comprising, in combination, frame means providing a window aperture of substantial area, means providing a plurality of light-polarizing elements at least substantially of the area of said aperture, a sheet of light-transmitting, resilient plastic material at least substantially of the area of said aperture and adapted when subjected to a predetermined compression in the plane thereof to function as a fractional wave retardation device, means for mounting said elements within said frame in substantially parallel relation, said sheet being positioned between and substantially overlying said polarizing elements and showing substantially no cold flow when subjected to compression, and releasable means for subjecting at least said sheet to said predetermined compression to vary the amount of light transmitted by said window.

9. A variable density window comprising, in combination, frame means providing a window aperture of substantial area, a plurality of sheetlike, light-polarizing elements at least substantially of the area of said aperture, means for mounting said elements in substantially parallel relation within said frame, a sheet of light-transmitting, resilient material having a high stress optical coefiicient and showing substantially no cold flow when under compression and being at least substantially of the area of said aperture and adapted when subjected to a predetermined compression in the plane thereof to function as a fractional wave retardation device, means for mounting said sheet between and overlying said polarizing elements, and releasable means for subjecting said sheet to such predetermined compression to vary the amount of light transmitted by said window.

10. As an article of manufacture, a sheet of light-transmitting, resilient, plastic material having a high stress optical coefficient and showing substantially no cold flow when under com pression and adapted when subjected to compression in the plane thereof to function as a fractional wave retardation device, and means providing a light-polarizing surface on each side of said sheet, each said surface being compressible with said sheet.

EDWIN H. LAND. 

